Add a submit old field to the getblocktemplate RPC during long polling

zebra-rpc/src/methods/get_block_template_rpcs.rs

@@ -349,7 +349,7 @@ where
 
             // The loop returns the server long poll ID,
             // which should be different to the client long poll ID.
-            let (server_long_poll_id, chain_tip_and_local_time, mempool_txs) = loop {
+            let (server_long_poll_id, chain_tip_and_local_time, mempool_txs, submit_old) = loop {
                 // Check if we are synced to the tip.
                 // The result of this check can change during long polling.
                 //
@@ -397,7 +397,22 @@ where
                 // - the server long poll ID is different to the client long poll ID, or
                 // - the previous loop iteration waited until the max time.
                 if Some(&server_long_poll_id) != client_long_poll_id.as_ref() || max_time_reached {
-                    break (server_long_poll_id, chain_tip_and_local_time, mempool_txs);
+                    let mut submit_old = client_long_poll_id
+                        .as_ref()
+                        .map(|old_long_poll_id| server_long_poll_id.submit_old(old_long_poll_id));
+
+                    // On testnet, the max time changes the block difficulty,
+                    // so old shares are invalid.
+                    if max_time_reached {
+                        submit_old = Some(false);
+                    }
+
+                    break (
+                        server_long_poll_id,
+                        chain_tip_and_local_time,
+                        mempool_txs,
+                        submit_old,
+                    );
                 }
 
                 // - Polling wait conditions
@@ -549,6 +564,7 @@ where
                 coinbase_txn,
                 &mempool_txs,
                 default_roots,
+                submit_old,
             );
 
             Ok(response)

zebra-rpc/src/methods/get_block_template_rpcs/types/get_block_template.rs

@@ -135,14 +135,28 @@ pub struct GetBlockTemplate {
     /// Zebra adjusts the minimum and current times for testnet minimum difficulty blocks,
     /// so we need to tell miners what the maximum valid time is.
     ///
-    /// This field is not in the Zcash RPC reference yet.
+    /// This field is not in `zcashd` or the Zcash RPC reference yet.
     ///
     /// Currently, some miners just use `min_time` or `cur_time`. Others calculate `max_time` from the
     /// fixed 90 minute consensus rule, or a smaller fixed interval (like 1000s).
     /// Some miners don't check the maximum time. This can cause invalid blocks after network downtime,
     /// a significant drop in the hash rate, or after the testnet minimum difficulty interval.
     #[serde(rename = "maxtime")]
     pub max_time: DateTime32,
+
+    /// > only relevant for long poll responses:
+    /// > indicates if work received prior to this response remains potentially valid (default)
+    /// > and should have its shares submitted;
+    /// > if false, the miner may wish to discard its share queue
+    ///
+    /// <https://en.bitcoin.it/wiki/BIP_0022#Optional:_Long_Polling>
+    ///
+    /// This field is not in `zcashd` or the Zcash RPC reference yet.
+    ///
+    /// In Zebra, `submit_old` is `false` when the tip block or max time is reached,
+    /// and `true` if only the mempool transactions have changed.
+    #[serde(rename = "submitold")]
+    pub submit_old: Option<bool>,
 }
 
 impl GetBlockTemplate {
@@ -156,6 +170,7 @@ impl GetBlockTemplate {
         coinbase_txn: TransactionTemplate<amount::NegativeOrZero>,
         mempool_txs: &[VerifiedUnminedTx],
         default_roots: DefaultRoots,
+        submit_old: Option<bool>,
     ) -> Self {
         // Convert transactions into TransactionTemplates
         let mempool_txs = mempool_txs.iter().map(Into::into).collect();
@@ -212,6 +227,8 @@ impl GetBlockTemplate {
             height: next_block_height.0,
 
             max_time: chain_tip_and_local_time.max_time,
+
+            submit_old,
         }
     }
 }

zebra-rpc/src/methods/get_block_template_rpcs/types/long_poll.rs

@@ -187,6 +187,23 @@ pub struct LongPollId {
     pub mempool_transaction_content_checksum: u32,
 }
 
+impl LongPollId {
+    /// Returns `true` if shares using `old_long_poll_id` can be submitted in response to the
+    /// template for `self`:
+    /// <https://en.bitcoin.it/wiki/BIP_0022#Optional:_Long_Polling>
+    ///
+    /// Old shares may be valid if only the mempool transactions have changed,
+    /// because newer transactions don't have to be included in the old shares.
+    ///
+    /// But if the chain tip has changed, the block header has changed, so old shares are invalid.
+    /// (And if the max time has changed on testnet, the block header has changed.)
+    pub fn submit_old(&self, old_long_poll_id: &LongPollId) -> bool {
+        self.tip_height == old_long_poll_id.tip_height
+            && self.tip_hash_checksum == old_long_poll_id.tip_hash_checksum
+            && self.max_timestamp == old_long_poll_id.max_timestamp
+    }
+}
+
 /// Update `checksum` from `item`, so changes in `item` are likely to also change `checksum`.
 ///
 /// This checksum is not cryptographically secure.